1. Field of the Invention
This invention relates to a process for producing a photoelectric conversion device, such as a solar cell.
2. Description of the Related Art
Photoelectric conversion devices comprising a photoelectric conversion layer and electrodes electrically connected to the photoelectric conversion layer have heretofore been used in use applications, such as solar cells. Heretofore, as the solar cells, Si type solar cells utilizing bulk single crystalline Si or polycrystalline Si, or thin film amorphous Si have been most popular. Recently, research and development have been conducted on compound semiconductor type solar cells that do not depend upon Si. As the compound semiconductor type solar cells, there have been known bulk types, such as GaAs types, and thin film types, such as CIS or CIGS types which are constituted of a Group-Ib element, a Group-IIIb element, and a Group-VIb element. The CI (G) S types are the compound semiconductors that are represented by the general formula of Cu1-zIn1-xGaxSe2-ySy, wherein 0≦x≦1, 0≦y≦2, and 0≦z≦1. In cases where x=0, the compound semiconductors are of the CIGS types. In cases where x>0, the compound semiconductors are of the CIS types. In this specification, both the CIS types and the CIGS types are often referred to as the CI (G) S types.
In the cases of the thin film types of the photoelectric conversion devices, such as the CI (G)S types, ordinarily, a transparent conductive layer (a transparent electrode) is formed on a photoelectric conversion layer with a buffer layer or a combination of the buffer layer and a window layer intervening therebetween. In the cases of the types described above, ordinarily, the buffer layer is formed by use of a chemical bath deposition technique (CBD technique).
Roles of the buffer layer are considered to be (1) prevention of recombination of photogenerated carriers, (2) matching of band discontinuity, (3) lattice matching, and (4) coverage of surface unevenness of the photoelectric conversion layer. In the cases of the CI (G) S types, and the like, ordinarily, the surface unevenness of the photoelectric conversion layer is comparatively large. Therefore, particularly, in order for the condition of (4) described above to be fulfilled satisfactorily, the CBD technique, which is a liquid phase technique, is considered to be preferable.
In cases where the buffer layer is formed with the liquid phase technique, and the film formation of the transparent conductive layer, the window layer, and the like, which are formed at stages after the formation of the buffer layer, is performed by use of vacuum processing, such as a sputtering technique, it is not possible to perform continuous film formation, such as roll-to-roll mode film formation, and therefore complicated processing is required. (Reference may be made to S. Ishizuka et al., “Efficiency Enhancement of Cu (In,Ga)Se2 Solar Cells Fabricated on Flexible Polyimide Substrates using Alkali-Silicate Glass Thin Layers”, Applied Physics Express, Vol. 1, 092303, pp. 092303-1-092303-3, 2008.) Also, in cases where the vacuum processing is performed after the liquid phase technique, particular surface treatment processing for the surface of a film having been formed often becomes necessary. Therefore, in order for the film formation processing to be simplified and in order for low-cost production to be achieved, it is preferable that the film formation at the stages after the formation of the buffer layer is performed entirely by use of the liquid phase technique.
As described above, as the film forming stage after the film formation of the buffer layer has been performed, the film forming stage for the transparent conductive layer is performed. In some cases, the film forming stage for the window layer is also performed. As the transparent conductive layer, a zinc oxide type of a transparent conductive layer has attracted particular attention for abundance of resources and a lower cost than ITO, which is popular currently. However, it is not always possible to form, with the liquid phase technique, the zinc oxide type of the transparent conductive layer which has good quality and which has a low electrical resistance value. The zinc oxide type of the transparent conductive layer is the electrically conductive zinc oxide thin film obtained with processing, wherein zinc oxide is doped with a dopant element having a higher valence number of ion than zinc and is thereby imparted with the electrically conductive characteristics. Therefore, for the formation of the electrically conductive zinc oxide thin film, it is preferable to use an electrolytic deposition technique (electrodeposition technique), which enables the doping of the dopant element in a high concentration. However, in the cases of the electrodeposition technique, it is necessary for an underlayer, on which the electrically conductive zinc oxide thin film is to be formed, to function as an electrode. Accordingly, in the cases of the photoelectric conversion device, wherein the underlayer is the non-conductive layer, such as the buffer layer or the window layer, it is not always possible to form the electrically conductive zinc oxide thin film that appropriately covers the underlayer and that has a low resistance.
Studies have been made on methods of forming an electrically conductive zinc oxide thin film that appropriately covers the underlayer and that has a low resistance. Each of Japanese Unexamined Patent Publication No. 2002-020884 and Japanese Patent No. 3445293 discloses a method of forming an electrically conductive zinc oxide film, wherein an initial layer of an electrically conductive zinc oxide layer is formed with sputtering film formation, and wherein an electrodeposition technique is thereafter performed. However, with the disclosed methods, all of the stages after the formation of the buffer layer are not performed entirely by use of the liquid phase technique.
A method of forming an initial layer for the electrodeposition technique by use of the CBD technique has been disclosed. The CBD technique is the liquid phase technique that enables the formation of the zinc oxide film on a non-conductive underlayer. However, since zinc oxide is a wurtzite crystal, in cases where a morphology control agent (such as an organic molecule) for growth control of a specific crystal face, or the like, is not used particularly in the CBD technique, a growth rate in the c-axis direction of the crystal is ordinarily quick, and the crystal is apt to grow in a rod-like shape. As a result, large rod-shaped crystals deposit, and a film is not formed. Even though a film is formed, a film structure, wherein a plurality of fine rod-shaped crystals stand side by side with a spacing being left therebetween, is obtained. It is thus not always possible to appropriately cover the underlayer.
As methods of controlling the crystal growth and forming a zinc oxide film which appropriately covers an underlayer, there have been proposed the methods, wherein a plurality of metal fine particles are imparted to the underlayer, and wherein a zinc oxide film is then formed with the CBD technique. In Japanese Patent No. 4081625, a method is disclosed, wherein an underlayer is catalyzed with an activator containing Ag ions, and wherein a zinc oxide film is then formed by use of a zinc oxide deposition solution. For example, in paragraph 0026 of Japanese Unexamined Patent Publication No. 2002-020884, a method is described, wherein an underlayer is catalyzed with an activator containing Ag ions, wherein zinc oxide is then deposited with an electroless technique, and wherein energizing processing is performed in a zinc oxide deposition solution by utilizing the thus obtained ZnO deposit as a cathode and utilizing a zinc plate as an anode, whereby ZnO is grown. Similar techniques are also described in J. Katayama, “Application of ZnO Prepared with Soft Solution Processing and Cu2O Semiconductor Thin Film to Optoelectronics”, Ritsumeikan University doctoral thesis, 2004; and H. Ishizaki et al., “Influence of (CH3)2NHBH3 Concentration on Electrical Properties of Electrochemically Grown ZnO Films”, Journal of The Electrochemical Society, Vol. 148, Issue 8, pp. C540-0543, 2001.
However, in cases where the electrically conductive zinc oxide film is formed ultimately by performing the steps up to the electrodeposition technique in accordance with each of the methods described in Japanese Unexamined Patent Publication No. 2002-020884; Japanese Patent No. 3445293; Japanese Patent No. 4081625; J. Katayama, “Application of ZnO Prepared with Soft Solution Processing and Cu2O Semiconductor Thin Film to Optoelectronics”, Ritsumeikan University doctoral thesis, 2004; and H. Ishizaki et al., “Influence of (CH3)2NHBH3 Concentration on Electrical Properties of Electrochemically Grown ZnO Films”, Journal of The Electrochemical Society, Vol. 148, Issue 8, pp. C540-0543, 2001, a specific resistance value of the obtained electrically conductive zinc oxide film is as high as approximately 7.8×10−3Ω·cm, which corresponds to a sheet resistance value of as high as approximately 200Ω/□, and a resistance value satisfactory for the electrode layer is not obtained. (The resistance value described above is cited from J. Katayama, “Application of ZnO Prepared with Soft Solution Processing and Cu2O Semiconductor Thin Film to Optoelectronics”, Ritsumeikan University doctoral thesis, 2004.) Also, in cases where the metal layer is used as the underlayer for the transparent conductive layer, since the metal layer affects a band gap, there is the risk that the device characteristics will become bad.